Apparatus and methods thereof for power consumption measurement at circuit breaker points

ABSTRACT

Apparatus and methods are provided for the measurement of power consumption at points of interest, such as circuit breakers, machines, and the like. Accordingly, means are provided for measurement of power consumption for each electrical sub-network that is controlled by a circuit breaker. Each apparatus is enabled to communicate its respective data, in an environment of a plurality of such apparatuses, to a management unit which is enabled to provide finer granularity power consumption profiles. Challenges of measuring relatively low supply currents, wireless operation in an environment of a large number of apparatuses, and self-powering are addressed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No. 14/586,605 filed Dec. 30, 2014, which is a continuation of U.S. patent application Ser. No. 12/760,867 filed Apr. 15, 2010, which claims the benefit of U.S. Provisional Patent Application No. 61/169,750 filed Apr. 16, 2009 and U.S. Provisional Patent Application No. 61/272,216 filed Sep. 2, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to the measurement of power consumption and more specifically to non-intrusive and self-powered measurement of electrical current flow through a power line to enable analysis of power consumption on a per circuit breaker basis.

2. Prior Art

In a typical electricity distribution system, power is provided through a main circuit breaker and a device for measurement of the power consumption of the entire electrical network connected thereto. However, typically, the main power line is then connected to a plurality of circuit breakers, each feeding a smaller section of the electrical network with its specific power requirements. The circuit breaker is adjusted to the amount of maximum current that may be used by this electrical sub-network. In industrial and commercial applications, hundreds of such circuit breakers may be installed, each controlling a section of the electrical network. Even in smaller locations, such as a house, it is not unusual to find tens of circuit breakers controlling various electrical sub-networks.

Non-intrusive measurement of current through a power line conductor has well known principles. A current transformer (CT) of sorts is created that comprises the primary winding as the power line conductor and the secondary providing an output current inversely proportionate to the number of windings. Typically such systems are used for measuring currents in very high voltage or current environments, for example, as shown in Gunn et al. in U.S. Pat. No. 7,557,563. These types of apertures are useful for main power supplies. Using such devices, or power meters for that matter, is deficient for the purposes of measuring relatively low currents in an environment of a plurality of circuit breakers. Providing wireless telemetry on a singular basis, such as suggested by Gunn et al., and other prior art solutions, suffers from deficiencies when operating in a noisy environment.

There is a need in the art that is now developing, resulting from the move toward energy conservation to enable analysis of power consumption on a finer granularity. This would require analysis on at least a per circuit breaker basis and such solutions are not available today. It would be further advantageous if a solution may be provided for installation in a circuit breaker closet for existing circuit breakers. It would be therefore beneficial to overcome the limitations of the prior art by resolving these deficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit breaker equipped with a compatible self-powered power sensor deployed in accordance with the invention.

FIG. 2 is a block diagram of a first embodiment of a self-powered sensor in accordance with the invention.

FIG. 3 is a circuit diagram of a first embodiment of the analog portion of the self-powered sensor in accordance with the invention.

FIG. 4 is a circuit diagram of a second embodiment of the analog portion of the self-powered sensor in accordance with the invention.

FIG. 5 is a circuit diagram of a third embodiment of the analog portion of the self-powered sensor in accordance with the invention.

FIG. 6 is a schematic diagram of a core with the secondary winding.

FIG. 7 is a schematic diagram of the two parts comprising the core.

FIG. 8 is a schematic diagram of a housing of a self-powered power sensor implemented in accordance with the invention.

FIG. 9 is a flowchart of the operation of a self-powered power sensor deployed in accordance with the invention.

FIG. 10 is a schematic diagram of a system configured in accordance with the invention.

FIG. 11 is a block diagram of a second embodiment of a self-powered sensor in accordance with the invention.

FIG. 12 is a circuit diagram of a fourth embodiment of the analog portion of the self-powered sensor in accordance with the invention.

FIG. 13 is a circuit diagram of a fifth embodiment of the analog portion of the self-powered sensor in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Apparatus and methods are provided for the measurement of power consumption at points of interest, such as circuit breakers, machines and the like. Accordingly, means are provided for measurement of power consumption for each electrical sub-network that is controlled by a circuit breaker. Each apparatus is enabled to communicate its respective data, in an environment of a plurality of such apparatuses, to a management unit which is enabled to provide finer granularity power consumption profiles. Challenges of measuring relatively low supply currents, wireless operation in an environment of a large number of apparatuses, and self-powering are addressed.

Reference is now made to FIG. 1 where an exemplary and non-limiting system 100 is equipped with a compatible self-powered power sensor (SPPS) 110 deployed in accordance with the invention. The SPPS 110 is designed to fit either above or below the circuit breaker 120 which is of standard size such that it fits into current circuit breaker closets without modification. The SPPS 110 housing is designed, as discussed in further detail below, to wrap around the power line 130 leading to or going out of the circuit breaker 120. The SPPS 110 is designed to enable easy installation at an existing location or otherwise during construction when the entire electrical network is put in place.

The SPPS contains an electrical circuit the exemplary and non-liming circuit 200 which is shown in block diagram form in FIG. 2. The circuit 200 comprises an analog section 210 that is coupled to a microcontroller 220. The analog section comprises a current transformer 212 to transform current from the power line, for example power line 130, to a lower current. The power sensed there from is used for two purposes, the first is to provide the power needed for the operation of the SPPS 110 and the second is to sense the actual power consumption of the load connected to the power line 130. The current to pulse converter (C2PC) 214 is used to generate periodically a pulse that is provided to the microcontroller unit (MCU) 220 and enables the measurement of the power consumption. The more frequent the pulses the higher the power consumption. The energy harvester 216 stores energy to be used as the power supply for the circuitry of SPPS 110. It is further enabled to receive a discharge signal from the microcontroller 220 to enable intentional discharge of the energy harvester 216 and prevent overcharge. In one embodiment of the invention a Zener diode (not shown) is used to clamp the voltage to the desired level thereby preventing overcharge.

The circuit 200 further comprises a MCU 220 that is comprised of several components. An analog-to-digital (A/D) converter 225 that is coupled to a signal processor 224 which is further coupled to the media access control (MAC) 222 that supports the communication protocol of the SPPS. The MAC 222 provides the data-link layer of the 7 layer standard model of a communication system. This involves the creation in hardware, software, firmware or combination thereof, of data frames, timing their transmission, received signal strength indication (RSSI), acknowledgements, clock synchronization etc. A counter 227 is excited by an interrupt signal received from the analog section 210 and enables the counting of the number of pulses that, as noted above, is proportionate to the power consumed for a given unit of time. Another A/D converter 226 is used to measure the output of the energy harvester 216, and in one embodiment, under control of MCU 220, to cause a discharge thereof as may be needed and as further explained below. In another embodiment, further explained herein below, it can be used to detect that the load connected to the measured power line was turned off. A memory 230 is coupled to the MCU 220 that can be used as scratch pad memory 230 as well as memory for storage of the plurality of instructions that when executed by the MCU 220 executes the methods discussed herein. Memory 230 may comprise random access memory (RAM), read only memory (ROM), non-volatile memory (NVM), other memory types and combinations thereof.

A radio frequency (RF) transceiver 240 is coupled to the MCU 220 and to an antenna 250 to provide one or two-way communication with a management unit, discussed in more detail below. In one embodiment of the invention the RF transceiver 240 supports transmission only, i.e., uplink communication. However, the RF transceiver 240 may comprise a receiver portion to support features such as, and without limitation, sensing for a carrier signal, clock synchronization, acknowledgement, firmware download, and configuration download. Typically, this should be an unlicensed industrial scientific medical (ISM) band transceiver, operative, for example and without limitation, at 2.4 Ghz. In one embodiment some form of spread-spectrum modulation technique may be used, for example and without limitation, direct sequence spread spectrum (DSSS), to enable better coexistence with other systems working in the same environment. The communication rate, discussed in more detail below, should be high enough to enable coexistence of a couple of hundred SPPSs in the same electrical closet. The power consumption of the RF transceiver 240 should be low enough to adhere with the energy harvesting limitations. Yet another requirement of the RF transceiver 240 is to support a communication range sufficient to operate in an electrical closet, e.g., 3-4 meters metallic reach environment. In another embodiment of the invention the range may reach up to a few tens of meters in an indoor environment. This enables the placing of SPPSs on individual devices, e.g., on machines in a production line of a factory, and a minimum number of bridge units in the area. The RF transceiver 240 preferably uses a standard PHY layer supporting, for example and without limitations, IEEE 802.15.4, and/or communication protocol, for example and without limitation, Zigbee®. Use of such standards enables easy integration with existing systems that already include wireless hardware, for example and without limitations, smart meters.

According to the principles of the invention, each time a pulse arrives from the C2PC 214 an interrupt signal is sent to the MCU 220. Responsive to receiving the interrupt pulse the MCU 220 wakes up and increases the counter 227 value. The energy stored in each pulse is larger than the energy required for wakeup and counting, hence enough energy is still available for charging the energy harvester 216 and/or enable transmission using the RF transceiver 240. The value of the counter 227 is proportional to the total charge which went through the primary line 130, i.e., current integrated over time. The value in the counter 227, as well as other parameters, are saved in the system's memory 230. The MCU 220 is enabled to periodically check for a condition to transmit. Such a condition may be one or more of the following conditions: sufficient amount of energy exists, upon a certain time lapse from a previous transmission, upon collection of certain data such as significant or otherwise interesting data, and other relevant conditions. According to the principles of the inventions detection of the existence of sufficient amount of energy for transmission, for example, through the A/D converter 226 connected to the energy harvester 216, it is possible to detect if its voltage reached a predetermined value.

Upon determination that a transmission is to take place the MCU 220 prepares a message to be transmitted. The message is typically a single packet of data that may contain various types of information and include the SPPS's unique identification (UID) which enables a management unit to positively associate the current data received with previous data handled by the management unit with respect of the SPPS. The value of counter 227, potentially multiplied by a calibration factor converting that value into a normalized charge unit relative to other sensors, for example, Ampere-Hour (AH), may be attached as part of the packet. The calibration factor may be programmed to the SPPS 110 in the NVM of memory 230 during calibration of the circuit 200, as part of final inspection during manufacturing. This ensures compensation against inaccuracies typical to the manufacturing process. The calibration factor may be a fixed value for all units or a specific calibration factor unique to each unit. The latter is useful for overcoming production tolerances of the SPPS. Other information may include, without limitations, various SPPS status information, hardware version, software version, alerts such as overload, phase information, average current, temperature, time duration information, power off indication, e.g., upon identification that the load was turned off, and other system parameters. Such parameters may be saved until such time of transmission in memory 230, and more specifically in a NVM portion of memory 230. A cyclic redundancy code (CRC) calculation, forward error correction (FEC), and/or data redundancy may be further added to a packet for data validation at the receiver side. In one embodiment, when the voltage of the harvesting circuitry is determined to be decreasing at a high rate, i.e., the power line load was turned off, the device transmits a message containing the last counter value as no energy may be available until the load is switched on again.

When condition(s) to transmit is (are) met, the MCU can implement a carrier sense multiple access (CSMA) mechanism for the purpose of collision avoidance. The following steps are therefore taken. First, the receiver of the RF transceiver 240 is switched on. Second the receiver senses whether there are currently other transmissions. This is particularly important in the environment in which the SPPS operates, which is an environment rich with SPPSs, possibly a few hundreds of them. Third, upon determination that the air is free, the receiver is disabled and the transmitter of the RF transceiver 240 is enabled for transmission to send the information message; otherwise, the receiver is disabled and the circuit 200 is caused to sleep for a random time interval, after which the circuit 200 wakes-up and the sequence of steps is repeated until the desired transmission is completed. In one embodiment of the invention, after completion of transmission the transmitter is disabled and the receiver is enabled to receive an acknowledgement signal from the management unit. In another embodiment of the circuit 200 the information messages are short enough and the intervals between transmissions are long enough so that collisions are highly unlikely. In such an embodiment the transmission of the information message may take place without pre-sensing of the air, thereby conserving energy. In yet another embodiment of the invention, after transmission the receiver is activated to receive a clock synchronization signal. This allows synchronization between the clocks of MCU 220 and the management server 1050 (see FIG. 10), and as further explained herein below.

In yet another embodiment of the invention sufficient amounts of energy are available in the circuit 200 for continuous or longer operation. This is possible in cases where the primary current is above a certain value. The MCU 220 can then remain on and perform signal processing on the non-rectified signal coming directly from the current transformer 212. The gathered information may be therefore transmitted more frequently. This is useful for example for measurements relating to peak values, average currents, phase calculation, frequency shift calculation, transient and irregular current over short period of time, and total harmonic distortion (THD). The reservoir voltage of energy harvester 216 is constantly measured by means of A/D converter 226 of MCU 220, in order to prevent overcharge. If necessary a discharge of the energy harvester 216 is performed through an I/O port. The voltage information further provides an indication of the available energy for keep-alive transmissions when no primary current exists. This may happen when the circuit breaker 120 tripped or was otherwise shutdown, or otherwise when no power is consumed by the electrical sub-network protected by the circuit breaker 120. In a further embodiment of the invention a 3-phase SPPS is implemented comprising three analog sections 210 each coupled to a single MCU 220, which is further coupled to the transceiver (240) and an antenna (250). The circuit is configured to handle three analog sections such that the single MCU 220 can handle the entire operation of a 3-phase SPPS. While a 3-phase SPPS is described it should be understood that a system comprising a plurality of analog sections may be implemented, for a single phase or multiple phase SPPS, thereby reducing the costs of such a multi-power-line-sensor SPPS.

Reference is now made to FIG. 3 depicting an exemplary and non-limiting circuit diagram 300 of a first embodiment of the analog portion 210 of the self-powered circuit 200 in accordance with the invention. The primary winding of the current transformer 310 is the power line 130 and its AC current induces voltage and current in the current transformer 310. The induced current resonates with the resonance capacitor 320 to produce sufficient voltage to pass through the diode bridge 330. In the case where Schottky diodes are used this voltage is approximately 0.3V. At the output of the diode bridge a rectified DC current is provided which charges the sense capacitor 340 until it reaches a certain threshold V_(1H). The comparator 360 detects V_(1H) on the sense capacitor 340, and produces a control signal to the DC/DC controller 370 which in turn activates the DC/DC switch 375 and boosts the voltage on the high capacitance reservoir capacitor 380 to a high voltage V₂, typically up to 12V. The control signal is also used as an interrupt to wake up the MCU 220 and raise a counter 227. Each discharge of the sense capacitor 340 represents a quantum of AH flowing through the main circuit. The frequency of the pulses is proportional to the primary current and the number of pulses is therefore proportional to the total AH flowing through the main circuit. The sense capacitor 340 is discharged through the DC/DC inductor 350 into the reservoir capacitor 380. The DC/DC control signal from the DC/DC controller 370 causes suspension of the discharge of the sense capacitor 340, once the comparator 360 detects a low threshold V_(1L), for example 0.5V, on the sense capacitor 340. The voltage of the reservoir capacitor 380 is regulated by the linear regulator 390 into a steady DC voltage, for example 3.3V or 2V as the case may be, which is supplied to the MCU 220, RF Transceiver 240, DC/DC controller 370 and the comparator 360.

Upon startup of circuit 300 the reservoir capacitor 380 is charged by the sense capacitor 340 until enough energy is stored in the reservoir capacitor 380 that provides a sufficient voltage to activate the comparator 360 and the DC/DC controller 370. The advantages of using a DC/DC converter are twofold: enabling the boosting of the reservoir capacitor 380 into a high voltage, hence enabling an energy reservoir sufficient for many RF transmission cycles; and, enabling a relatively low V_(1H)/V_(1L) range, hence enabling the circuit 300 to operate at very low primary currents by producing, typically, only up to 1V at the sense capacitor 340. The voltage of the reservoir capacitor 380 is provided to the A/D converter 226 of the MCU 220 thereby enabling an intentional discharge to prevent overcharge. Discharge is achieved by the MCU 220 through control of the I/O terminal of transistor 395. In another embodiment, as also previously discussed, a Zener diode (not shown) is used for the purpose of overcharge control. In another embodiment the A/D converter 226 is configured to detect if the load connected to the primary line was turned off and hence consumes zero current. In this case the voltage on the reservoir capacitor 380 drops at a high rate as no energy is supplied to the circuit 200. The transmitter therefore transmits a single message indicating that power was turned off. The message may further contain the last counter value sampled prior to the reservoir energy depletion. The non-rectified output of the current transformer 310 is coupled to the A/D converter 225 of the MCU 220, for example using a small sense resistor (not shown) thus enabling additional signal processing and measurements when enough energy exists in the circuit 300. For example, and without limitations, phase measurement or detection of irregular behavior may be achieved at such times. By limiting the voltage of the sense capacitor, the voltage on the CT 310 coil is kept low hence the magnetic core can be operated below its natural saturation point which increases the measurement accuracy.

The resonance capacitor 320 resonates with the current transformer coil in order to produce a sufficiently large voltage to pass through the diode rectifier. Since the magnetization curve of a typical core is non linear at low primary currents, the effective inductance of the core varies with primary current. In one embodiment of the invention, it is beneficial to select the resonance capacitor's value so that maximum resonance is achieved at low primary currents. This produces the required voltage swing to pass through the diode bridge even at very low primary currents.

FIG. 4 depicts an exemplary and non-limiting circuit diagram 400 of a second embodiment of the analog portion 210 of the self-powered sensor 110 in accordance with the invention and wherein transformer 410, resonance capacitor 420 and bridge rectifier 430 correspond to transformer 310, resonance capacitor 320 and bridge rectifier 330 of FIG. 3. The circuit is simpler than the circuit 300 as it does not use a DC/DC controller. In this embodiment, when the sense capacitor 440 reaches 3V, the comparator 450 activates the switches 452 and 454. Activation of the switch 452 enables charging the reservoir capacitor 470 directly from the sense capacitor 440. The switch 454 changes the comparator 450 thresholds. When the sense capacitor 440 is discharged to 2.2V the comparator disengages the capacitors, i.e., transfer of energy to the reservoir capacitor 470 ceases. The voltage on the reservoir capacitor 470 is regulated to, for example, 2V, the voltage which is the V_(CC) voltage of the MCU 220 and the RF transceiver 240. In many cases, the internal voltage regulator of the MCU 220 may be used since the voltage range is minimal. When the voltage of the reservoir capacitor 470 voltage is above, for example, 2V, the MCU 220 is capable of waking up and drawing current for pulse counting and transmission as described above. The MCU 220 enables the reservoir capacitor 470 to be charged to a peak voltage of, for example, 2.2V. Overcharge is prevented by intentional discharge as described in the previous embodiment. In this case, since no DC/DC is used, it is critical to keep the voltage of the reservoir capacitor 470 lower than the low threshold of the sense capacitor 440, for example, 2.2V, in order to prevent charge from flowing backwards. In another embodiment, as also previously discussed, a Zener diode (not shown) is used for the purpose of overcharge control. An optional small auxiliary battery 460 is used in order to feed the comparator 450, provide initial operating energy when the reservoir capacitor 470 is not fully charged, and provide enough energy for low frequency, for example once per day, keep-alive transmissions when no primary current exists. Keep alive transmissions are important in order to notify the system of the existence of the sensor even when no primary current exists.

FIG. 5 depicts an exemplary and non-limiting circuit diagram 500 of a third embodiment of the analog portion 210 of the self-powered sensor 110 in accordance with the invention and wherein transformer 510, resonance capacitor 520 and bridge rectifier 530 correspond to transformer 310, resonance capacitor 320 and bridge rectifier 330 of FIG. 3 and transformer 410, resonance capacitor 420 and bridge rectifier 430 of FIG. 4. In this embodiment of the analog portion 210 there is only one large sense capacitor 540 and no reservoir capacitor nor a DC/DC controller. The reason for using lesser components in the circuits shown in FIGS. 4 and 5 is to reduce the component count and thereby reduce the bill-of-materials (BOM) of the solution. In the circuit 500 the sense capacitor 540 also functions as the energy source for, typically, a single transmission. Therefore, the sense capacitor 540 of this embodiment is designed with a rather large capacitance, for example 1 mF. According to the principles of operation of the circuit 500 the comparator 550 detects when the sense capacitor 540 is charged, for example, up to 4V, and opens the switch 552 towards the linear regulator 570. The linear regulator 570 provides a regulated voltage, for example a 3V output, thereby allowing the MCU 220 to draw current resulting in discharge of the sense capacitor 540. Due to the activation of switch 554, discharge to a lower reference voltage, for example 3V, is detected by the comparator 550 and discharge is stopped. The MCU 220 is enabled to perform operations which discharge the sense capacitor 540 to perform the counting operation and transmission when needed. The MCU 220 is further enabled to measure the voltage of the sense capacitor and discharges it down to a lower voltage, for example 3V, intentionally when performing operations that do not consume the entire energy. An optional battery 560 is used to provide a reference voltage to the comparator 550, as well as to allow keep-alive transmissions when the primary current is below a minimum detectable current. In another embodiment, as also previously discussed, a Zener diode (not shown) is used for the purpose of overcharge control. In another embodiment, as also previously discussed, a linear regulator is not used and the MCU's internal regulator regulates the input voltage.

In another embodiment of the invention, power measurement is done by measuring the voltage change rate on the sense capacitor, e.g., capacitors 540, 440 or 340. The sense capacitor voltage is measured by A/D 226. The MCU 220 then lets the capacitor discharge through a resistor, for example resistor 395, for a fixed period of time, during which the MCU 220 can be set to a low power mode. The voltage level of the sense capacitor is measured after the elapse of the fixed period of time, and the voltage difference (ΔV) between the two measurements is calculated. ΔV consists of a negative fixed part, i.e., the voltage discharge through resistor 395, plus a positive variable part proportionate to the charge rate of the capacitor due to the primary current flow.

Key to the operation of the SPPS 110 is that it is capable of addressing several critical challenges to its successful operation. Three key issues are the minimum power detection of the current transformer 212, the power balance of the circuit 200, and wireless coexistence in an environment of a plurality of SPPSs 110 that may include several hundreds of SPPSs. In order for an SPPS 110 to be a useful device it is necessary that it be capable of detecting as low as possible currents flowing through the primary lead 130. The design must take into consideration the limited space typically available for an apparatus such as, but not limited to, SPPS 110 that must fit dimension restrictions of the circuit breaker 120. In other embodiments of the invention other size restrictions may apply, however these should not be viewed as limiting the scope of the invention. Inductance of the secondary winding is approximately:

$L = \frac{\mu_{0}\mu_{r}N^{2}A}{1}$

Where N is the number of windings, μ_(r) is the relative permeability of the magnetic material, such as, and not limited to, strip wound iron, μ₀ is the permeability of free space, A is the cross section of the core, further discussed with respect of FIGS. 6 and 7 below, and l is the effective length of the core. For N=1500, μ_(r)=1000, μ₀=4π10⁻⁷, A=40 mm², and 1=20 mm, the inductance is L=5.5 Hy. The current ratio between the secondary current I_(s) and the primary current I_(p) is approximately, for an ideal transformer, I_(p)/I_(s)=N. The voltage on the secondary coil is given by V_(s)=I_(s)ωL=I_(p)ωL/N, and at f=50 Hz ω=2πf=314 rad/sec. Therefore, V_(s)=I_(p)ωL/N=1.15 I_(p). Assuming a 1V drop over the diode rectifier, for example diode rectifier 330, and charge voltage of 1V then at least 2V are needed in order for the system to operate. Hence, there is a minimum detectable current of 2/1.15=1.7 A peak=1.2 A RMS. Using the resonance capacitor, for example resonance capacitor 320, the impedance is decreased by a factor of 1/(X_(L)−X_(C)) where X_(L) is the impedance of the core and X_(C) Is the impedance of the resonance capacitor. Taking an accumulative tolerance of ±20% for the capacitance and inductance, results in a worst case of 40% increase in signal, and hence the minimum detectable current is, in this exemplary case, 1.2×0.4=0.48 A, which represents a minimum detectable power of 105 VA at 220V. At 110V 60 Hz, the minimum detectable current in the exemplary case is 5/6×0.48=0.4 A and a minimum detectable power of 44 VA. Since L is proportional to N² and to A and V is proportional to 1/N, the minimum detectable current may be decreased by increasing either N or A. However, it is essential to ensure that the entire core, and its respective secondary winding, fit in the size constraints of SPPS 110, and an increase of N or A may have a material effect thereon.

Furthermore, to make the SPPS 110 an operative device it is essential to ensure that a sufficient amount of power may be made available through the operation of the circuits discussed hereinabove. Following is an exemplary and non-limiting analysis thereof. Firstly it is essential to understand the energy requirements of each of the key components: the transmission cycle, the counting cycle and the logic operation. Failure to address these issues may result in non-operative circuits. In all cases the assumption is for a 3V operation. For the transmission cycle a transmission current of 20 mA is used for a period of 5 mSec. A processing current of 1 mA is used during a 10 mSec period of wakeup and processing. Therefore the total energy requirement for the transmission cycle is: 3V×(20 mA×5 msec+1 mA×10 msec)=0.33 mJ. For the counting cycle a processing current of 1 mA is used for a wakeup and processing period of 5 mSec. Therefore the energy requirement for this counting cycle is: 3V×1 mA×5 msec=15 μJ. Lastly, the logic operation requires a continuous current of 50 μA, resulting in a continuous power consumption of: 3V×0.05 mA=150 μM. The total energy has to be supplied reliably by the power supply circuit, for example, circuit 300. It is therefore necessary that the sense capacitor, for example sense capacitor 340, and the reservoir capacitor, for example reservoir capacitor 380, provide sufficient energy for the performance of the desired operations. The above assumptions are typical for common low power MCUs and radio frequency integrated circuits (RFICs).

To address the energy balance of the circuit 200 it is necessary to ensure that the sense capacitor, for example sense capacitor 340, is capable of supplying sufficient energy for the counting cycle and that the reservoir capacitor, for example reservoir capacitor 380, is capable of supplying enough energy for several transmission cycles. Both are addressed in the following exemplary and non-limiting calculations. If the sense capacitor C₁ is equal to 1 mF and is charged to V₁=1V and discharged to V₂=0.5V, then the total discharge energy is: E=0.5 C₁×(V₁ ²−V₂ ²)=375 μJ. It has been shown hereinabove that the counting cycle requires 15 μJ which is less than 3% of the available energy. The remaining energy is accumulated for the purposes of transmission, for example, in the reservoir capacitor. Assuming a reservoir capacitor, for example capacitor 380, having a value of 0.375 mF, the capacitor being charged to V₁=5V and discharged to V₂=3V, then the total energy is: E=0.5 C₂×(V₁ ²−V₂ ²)=3 mJ. A previous calculation has shown that the transmission cycle consumes around 0.33 mJ and hence roughly nine transmission cycles are possible under these conditions. Now it is possible to determine the number of counting cycles it takes to charge the reservoir capacitor with the required amount of energy. The available energy is 360 μJ and with a 50% DC/DC controller efficiency there are 180 μJ at every sense capacitor pulse. By dividing the amount of energy required for several transmission cycles, e.g., 3 mJ, by the amount of energy charged each cycle, e.g., 0.18 mJ, it is determined that approximately 17 sense capacitor cycles are needed to charge the reservoir capacitor with the required energy.

In order to ensure proper operation of the circuit 200 it is necessary to ensure a positive energy balance for continuous system operation even at the lowest primary currents. It is therefore necessary to calculate the power in to the system versus the power out of the system, the later having to be smaller than the earlier. For the power in, at a primary current I_(p)=0.5 A and N=1500 results in a secondary current of I_(s)=0.33 mA. Using the same figures as above, i.e., a sense capacitor of 1 mF, discharge voltage down to 0.5V and charge voltage of 1V, the charge time is T=CΔV/I_(s)=1.5 Sec. The available energy of 375 μJ therefore provides 375/1.5=0.25 mW. Assuming 80% DC/DC efficiency, the available power in is 200 μW. The power out is a combination of the continuous logic operation, the counting process and the transmissions. The continuous logic operation requires 150 μW as shown above. The counting processing requires 15 μJ for a period of 375 mSec which is equivalent to 40 μW. Assuming a transmission once every one minute then 360 μJ are required every 60 seconds which are 6 μW. The total power consumption is therefore 194 μW which is less than the 200 μW available as explained herein above. It should be noted that a higher primary current results in an improved power balance that enables an increase of the transmission frequency, performing continuous signal processing, storing energy for times when no primary current exists, and combinations thereof.

FIGS. 6 and 7 show schematic diagrams 600 and 700 of a core with the secondary winding and the core separated into two parts. The core is comprised of two parts 610 and 620 that are separable from each other, however, as shown in FIG. 7, are designed so as to ensure that when they are assembled they provide good magnetic flow through the core by reducing the air-gap between the two parts to minimum, for example 10 μm. While an exemplary shape of the two portions of the core is shown these are merely for explanation purposes and other designs are possible to achieve the required results. It is essential, as explained herein above, that the core fit in the dimensions allotted in the SPPS 110 so that it can properly fit in an electricity closet in conjunction with a circuit breaker. The secondary windings 630 of the current transformer 212 are wound on one of the sections of the core, for example, section 610 which is the stationary section that is placed in the exemplary and non-limiting housing 800 shown with respect of FIG. 8. In this example, these may be two windings connected in series, of two independent secondary windings (see FIG. 6). The moveable section of the core, for example section 620, is placed in section 810 of the housing 800 which is separable from section 820 of the housing, in which section 610 is placed. When separating section 810 from section 820 it is possible to place them around power line 130 so that when the sections 810 and 820 are reconnected the power line 130 is placed within the core perimeter and out opening 830, thereby completing the current transformer 212. Each SPPS 110 is assigned a unique identification (ID), for example a MAC address that may be 16 bytes in length, that is placed on the housing 800 at, for example, location 840. At installation of the SPPS the MAC address is read by a technician installing the system for configuration purposes. In one embodiment machine readable code is provided, e.g., barcode, to enable automatic reading using a reader. While a core comprising of two sections is described hereinabove, it should be noted that other implementations for a core are possible without departing from the scope of the invention. In one embodiment a single section core is used and in such a case the primary line must be inserted through the hole in the core. It may require disconnection of the line and threading it through the core for mounting the SPPS device.

An exemplary and non-limiting flowchart 900 depicted in FIG. 9 describes the operation of a SPPS deployed in accordance with the invention. In S910 the SPPS, for example, SPPS 110, checks if counting pulse was received and if so execution continues with S920; otherwise, execution continues with S910. In S920 a count is performed in accordance with the principles described herein above, which may include the discharge of the sense capacitor, for example capacitor 320. In S930 it is checked whether there is sufficient energy to perform a transmission and is so execution continues with S940; otherwise, execution continues with S910. In S940 it is checked whether it is time to transmit by the SPPS 110 and if so execution continues with S950; otherwise, execution continues with S910. In S950 SPPS 110 senses the environment for another transmission to avoid transmission collisions as discussed herein above. In S960 it is checked if it is possible to transmit and if so execution continues with S980; otherwise, in S970 a random wait period is determined and execution then continues with S930. In S980 the information gathered by the SPPS 110 is transmitted, the information transmitted contains data as discussed herein above. In S990 it is checked whether the operation should continue and if so execution continues with S910; otherwise, execution terminates. An optional step may be added after transmission is complete for the purpose of reception of feedback information from the unit receiving the information sent by the transmitter. Such feedback information may include, but is not limited to, acknowledge information and/or synchronization information.

Reference is now made to FIG. 10 where an exemplary and non-limiting system 1000, configured in accordance with the principles of the invention, is shown. The system comprises a plurality of SPPS 1010 communicatively coupled to a communication link 1020. The SPPS 1010 may be placed in an electrical closet before or after respective circuit breakers or, at the input to specific power consuming units. The management server is equipped with a transceiver enabling the communication with the plurality of SPPS 1010 using one or more of the communication schemes discussed herein above. The communication bridge 1020 is configured to communicate with those SPPSs 1010 it is configured to operate with, using for identification their respective MAC addresses. The communication bridge 1020 is coupled to a network 1030 which may be, but is not limited to, a local area network (LAN), a wide area network (WAN), a metro area network (MAN), the Internet, the world wide web (WWW), the likes and combinations thereof. The communication link can be, but is not limited to, a WLAN (Wireless LAN), for example 802.11 also known as WiFi, a wireless sensor area network, for example 802.15.4 also known as Zigbee®, power line communication (PLC), or a cellular to modem network such as GPRS or CDMA. In one embodiment of the invention the communication bridge aggregates the data from the plurality of sensors 1010-1 to 1010-N prior to sending it to the network. To the network there are coupled a database 1040 to accumulate data collected by the communication bridge 1020. The communication bridge 1020 may be placed in each closet and aggregate a plurality of SPPSs 1010 communications. In one embodiment the communication bridge 1020 is responsible for the phase calculation discussed in more detail herein below. Further coupled to the network is a management server 1050 that based on the data accumulated in database 1040 may provide a client 1060 processed information respective of the collected data as well as communicate with other application software, for example building management systems (BMSs). In one embodiment of the invention the minimum number of winding in the secondary coil is 500.

In one embodiment of the invention the communication bridge 1020 is enabled to provide information with respect to a phase and enable the system to calculate a phase shift. Knowledge of the phase shift between current and voltage is used to calculate the power factor (cos φ), hence determine more accurately the real active power flowing through the power line. When it is determined that there is sufficient energy in energy reservoir 216 then MCU 220 may become operative in continuous mode, for as long as such sufficient energy is available, or until operation is complete. Using AD converter 225 MCU 220 detects the peak current of the current transformer 212. The time of the peak relative to a clock synchronized between the sensor and the bridge unit is recorded and, when appropriate, transmitted to the communication bridge 1020, according to the principles discussed hereinabove. Communication bridge 1020 is further enabled to detect the peak of the power supply voltage nearest to the sensors by at least a peak detector (not shown) coupled to the communication bridge 1020 and to a reference power line. The time of the peak of is recorded by the communication bridge 1020 continuously. As the clocks of the communication bridge 1020 and circuit 200 are synchronized, as further discussed hereinabove, it is now possible for the communication bridge 1020, upon receiving information from the circuit 200 respective of the measure peak and time, to determine the phase shift between the reference power line voltage and the current measurement made by the circuit 200. It should be noted that the use of a peak detector enables the system to become agnostic to the differences in the utility grid frequency, e.g., 60 Hz for the USA versus 50 Hz in Europe, as well as to any other error or change in the supply voltage frequency.

Reference is now made to FIG. 11 where an exemplary and non-limiting second embodiment of a SPPS 1100 is shown. A key difference may be observed in the microcontroller 220 that does not receive a pulse as an interrupt signal as was shown in the previously described embodiments, for example in FIG. 2. Similar components to those of FIG. 2 are not further discussed herein, unless necessary for clarity. The notable change is in the analog section 1110 that comprises a current transformer 212, an energy harvester 216, a switch 1114 and a sense resistor 1112. In normal operation the switch 1114 is positioned to enable energy harvesting by the energy harvester 216. Periodically, for example under the control of the microcontroller 220, the switch 1114 is activated to short the secondary winding of transformer 212 through the sense resistor 1112, typically having a low resistance. The voltage on the sense resistor 1112 is sampled by the ADC 225. In order for the system 1100 to identify a voltage peak the process is repeated several times in each cycle. The switch 1114 is toggled between the two positions to enable energy harvesting most of the time in a first position, and measurement of the voltage periodically when in the second position. The sampling is averaged over a number of cycles and divided by the resistance value of the sense resistor 1112 to provide the current value. The current value is then multiplied by a time interval to obtain the total charge value, for example, in Ampere Hours. A calibration factor, as discussed herein above, can also be used with respect of system 1100.

The analog section may be implemented as shown in the exemplary and non-limiting circuit diagram 1200 of FIG. 12. Normally, the transistor switches 1210 and 1220, connected between the resonance capacitor 320 and the bridge rectifier 330 are off, so that the harvesting capacitor 380 is charged. The voltage of the harvesting capacitor 380 is limited to avoid overcharge as discussed in detail herein above with respect to other embodiments of the invention. From an energy harvesting point of view, FIG. 12 represents an embodiment close to the one shown in FIG. 5 but embodiments similar to the ones shown in FIGS. 3 and 4, in terms of the harvesting circuitry, are also possible. To perform a measurement the microcontroller 220 switches the transistor switches 1210 and 1220 using their respective I/O ports. According to the principles of the invention transistor switches 1210 and 1220 are operated simultaneously in opposite phases. Although measurement is preformed on a single resistor 1215 rather than two, the use of the two switches and two resistors is in order to prevent DC load on the transformer 212. This is required to avoid saturation and distortion of the measurement results. It would be appreciated by those skilled in the art that one switch conducts in the positive part of the cycle, and the other switch conducts in the negative part of the cycle. It should be noted however that topologies using a single switch which can symmetrically conduct in both directions are possible, for example, by using a pair of MOSFET transistors connected in series. When the switches are active the current flows through the appropriate sense resistor instead of charging the harvesting capacitor 380. According to the invention, the sense resistors have a low impedance relative to the self resistance of the transformer coil. This enables a close to short circuit current flow, keeping the voltage across the resistor low enough thus maintaining minimal flux across the core and avoiding saturation of the transformer 212. In one embodiment of the invention, after switching on the sense resistors, the MCU 220 waits a certain time interval, typically a couple of hundreds of milliseconds, or switch to an off/power save mode, before performing the measurement, in order to allow for the resonance capacitor to discharge. This ensures high accuracy and better linearity of the measurement results. In accordance with the principles of the invention, in cases where it is possible to use two coils, a first secondary coil used to measure the voltage using the ADC 225 while the second secondary coil (see prior descriptions of FIGS. 6 and 8) is used for the purpose of energy harvesting, thereby eliminating the need for switching at the expense of a potential increase in size of the SPPS. The value of the sense resistor may be easily calculated. Assuming the SPPS is designed for a maximum primary current of 30A then with N=1000 the maximum short circuit current of the secondary winding would be 30 mA. If the maximum input to the ADC 225 is 1V then the sense resistor 1112 is to be 30Ω. The resistance of a thin, e.g., 0.1 mm, copper wire with 1000 windings at typical dimensions of the SPPS is approximately 100Ω. Referring to the energy balance calculation explained hereinabove with respect to different embodiments, a similar amount of energy calculated before for the purpose of pulse counting, can be used here for the purpose of A/D activation and sampling, thus this embodiment does not significantly differ from the previous ones in terms of energy consumption. Therefore a sufficient amount of energy is available for proper system operation even when a very low primary current exists.

In yet another exemplary embodiment of the analog section circuit 1300, shown in FIG. 13, a voltage doubler 1340 is used. Transistor switches 1310 and 1320 correspond to transistor switches 1210 and 1220 of FIG. 12. In fact, the bridge rectifier described herein above with respect to all of the other embodiments can be replaced by a voltage multiplier. A person skilled in the art would readily note that the voltage multiplier may be a voltage doubler, tripler, quadrupler or any other type of passive voltage multiplier topology, without departing from the scope of the invention. The exemplary and non-limiting circuit 1300 shows a simple implementation of a voltage doubler 1340 coupled to pass capacitor 1330. The voltage on the harvesting capacitor 380 is double the voltage on the transformer 310 after resonance. In some cases the use of a voltage multiplier is advantageous at the lower current range. Also, specifically referring to the sense resistor topology, the voltage multiplier simplifies the grounding of the circuit as a common ground can be connected to the harvesting capacitor and the sensing resistor, whereas when using the bridge rectifier a differential voltage measurement needs to be made.

The principles of the invention, wherever applicable, are implemented as hardware, firmware, software or any combination thereof. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPUs”), a memory, and input/output interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. The circuits described hereinabove may be implemented in a variety of manufacturing technologies well known in the industry including but not limited to integrated circuits (ICs) and discrete components that are mounted using surface mount technologies (SMT), and other technologies. The scope of the invention should not be viewed as limited by the types of packaging and physical implementation of the SPPS 110 or the communication bridge 1020.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 

What is claimed is:
 1. An apparatus comprising: at least one analog section comprising a current transformer comprising a transformer core configured to mount around an alternating current (AC) power line making it a primary winding of the current transformer, the analog section for harvesting energy from a secondary winding wound around the transformer core, and storing it for use by components of the apparatus, a switch for connecting the secondary windings terminals between sampling of a current by a sense resistor and harvesting energy, and a resonance capacitor coupled in parallel to the secondary winding of the current transformer to resonate with the secondary winding inductance at a primary frequency, the resonance capacitor's value being selected so that maximum resonance is achieved at low primary currents; a microcontroller coupled to the at least one analog section to receive harvested energy, to at least an analog signal responsive to the alternating current in the AC power line by the sense resistor, and to at least the switch, wherein the microcontroller is configured to control the switch to connect the secondary windings terminals between sampling of a current by a sense resistor and harvesting energy; a memory coupled to the microcontroller; and a transmitter enabled to periodically transmit, under the control of the microcontroller, information responsive to the power consumption of a load connected to the power line; wherein the current through the AC power line, when present, being of a fixed frequency; wherein the current flowing through a circuit breaker connected to the AC power line ranges from a maximum current to low currents and is the fixed frequency of the current flowing though the AC power line; and wherein the resonance capacitor's value being selected to achieve resonance at the fixed frequency of the current flowing through the circuit breaker, at a low range of current through the circuit breaker, and at a low non-linear range of a magnetization curve of the transformer core.
 2. The apparatus of claim 1, wherein the sense resistor is coupled to a secondary winding of the current transformer, and has a resistance which is smaller than the resistance of the secondary winding.
 3. The apparatus of claim 1, further comprising: a diode bridge coupled in parallel to the secondary winding of the current transformer.
 4. The apparatus of claim 1, further comprising: an enclosure to house therein the at least one analog section, the microcontroller, the memory and the transmitter, wherein the enclosure is designed to mount around a power line connected in proximity of a circuit breaker of a plurality of circuit breakers in a circuit breakers' closet.
 5. The apparatus of claim 4, wherein the switching comprises use of a first switch operative in the positive phase of the AC cycle and a second switch operative in the negative phase of the AC cycle.
 6. The apparatus of claim 1, further comprising: a voltage multiplier coupled in parallel to the secondary winding of the current transformer.
 7. The apparatus of claim 1, wherein the sense resistor is connected to both the secondary winding of the current transformer and the resonance capacitor, and has a resistance which is smaller than the resistance of the secondary winding.
 8. The apparatus of claim 1, further comprising: a diode bridge coupled in parallel to the secondary winding of the current transformer and in parallel to the resonance capacitor.
 9. The apparatus of claim 1, further comprising: a voltage multiplier coupled in parallel to the secondary winding of the current transformer and in parallel to the resonance capacitor.
 10. The apparatus of claim 1, wherein the fixed frequency is one of: 50 Hz or 60 Hz.
 11. A method for sensing power consumption by an analog section of a self-powered power sensor (SPPS), comprising: controlling a switch by a microcontroller of the SPPS to alternate between current sensing using a sense resistor and power harvesting; activating a transmitter of the SPPS for transmission; transmitting information respective of power sensed by the sense resistor upon determination that the SPPS has accumulated sufficient power for transmission; and deactivating the transmitter of the SPPS; the SPPS comprising: an along section comprising a current transformer, a sense capacitor coupled to a secondary winding of the current transformer, the switch and the sense resistor, the microcontroller coupled to the analog section, a memory coupled to the microcontroller, and the transmitter coupled to the microcontroller; wherein the sense capacitor is selected to resonate with the secondary winding at a primary frequency, the resonance capacitor's value being selected so that maximum resonance is achieved at low primary currents; wherein the sense capacitor is coupled in parallel to the secondary winding of the current transformer; wherein the primary frequency is a fixed frequency of a primary winding current of the current transformer, the primary winding being an alternate current power line; and wherein a capacitance of the sense capacitor being selected to achieve resonance at the fixed frequency of the primary winding current of the current transformer, at low currents of the primary winding of the current transformer, and at a low non-linear range of a magnetization curve of a transformer core of the current transformer.
 12. The method of claim 11, wherein the fixed frequency is one of: 50 Hz or 60 Hz. 